Managing Fish Populations with Size Limits

Dr. Mike S. Allen

Department of Fisheries and Aquatic Sciences

The University of Florida

One of the most common management questions that fisheries biologists hear from anglers is "why donít you put a size limit on this lake?". Size limits are used by fisheries managers to increase the number of fish in a lake, increase the number of large fish, or to provide fishing experiences desired by anglers such as catching a trophy fish. Size limits have the potential to increase the number of large fish in a lake (bass or speckled perch). However, size limits will not help every fish population, and they have the potential to reduce angler catch and harvest without any benefits of providing bigger fish. In this article, I will discuss factors that fisheries managers evaluate when considering a size limit on a fishery.

So what factors determine the success of a size limit? The three components that are critical are G, R, and M of the fish population (Growth, Recruitment, and Mortality). G, R, and M are measured by fisheries biologists and usually determine the success or failure of a length limit on any given lake or any given species. So letís talk about G, R, and M and describe how they affect fish populations.

Growth rates determine how long a fish takes to reach a given size, and they can be highly variable among water bodies. For example, in some lakes bass may attain 3 pounds in 1.5 to 3 years, which is considered rapid growth. In fast growing speckled perch populations, fish may reach 10 inches by age 2 or 3. Regulations on lakes with rapid growth will protect the fish from harvest, and they will quickly grow to a size preferred by anglers. Alternately, slow growing bass populations may not attain 3 pounds until age 4-6, and slow growing speckled perch may take 4-5 years to reach 10 inches. Stringent regulations on a slow-growing fish populations will protect fish from harvest and may cause bass abundance to increase. This can result in even slower growth due to low food availability per individual. Extreme cases of slow growth often occur in farm ponds where bass may nearly stop growing, resulting in very few large fish and many fish in the 10-12 inch size groups. Ponds with very slow growth would benefit from harvest by reducing bass abundance, increasing the amount of prey available per fish, which would improve growth rates. Slow growth also occurs for both bass and speckled perch in larger lakes, and size limits on these populations may be detrimental to production of large fish if growth rates are slow.

Mortality is measured as the percent of fish that die each year. Fisheries biologists usually divide total mortality into fishing mortality (the percent that die from harvest and hooking mortality) and natural mortality (the percent that die due to causes other than fishing). Size limits are the primary tool to manage mortality rates via changes in fishing mortality. Size limits will improve the number of large fish in lakes with high fishing mortality, assuming that growth rates are adequate for fish to reach a preferred adult size. However, in lakes where fishing mortality is low due to low fishing effort or high rates of catch-and-release, size limits will usually not improve the number of large fish in the population.

Recruitment is the number of young fish that survive to adulthood (usually one year old) each year. Recruitment also varies widely from lake to lake and across years in the same lake. Lakes with 20-40% coverage of aquatic plants usually have high bass recruitment, whereas lakes with little shoreline cover often have low to intermediate recruitment. Speckled perch recruitment is sometimes influenced by water levels, with good recruitment in high-water years. Lakes with high recruitment are often the best fisheries if growth rates are rapid and mortality rates are relatively low. This causes many fish to survive to a preferred size by anglers. However, high recruitment can be detrimental if growth rates are slow, because slow growing populations will not benefit from the addition of many more hungry mouths to feed! Populations moderate to fast growth rates and high recruitment are often managed with slot limits, which encourage anglers to remove small fish and sustain adequate growth of fish in the protected slot.

Recruitment can fluctuate substantially on a given lake from year to year. Reductions in water levels often reduces the amount of cover for young fish and may result in poor survival, whereas period of high water may inundate shoreline cover and produce very strong year classes. Many of these fluctuations are out of the fishery biologistsí control but will strongly affect the number of adult fish that are present in the population for several years. Most lakes exhibit recruitment fluctuations across years due to changing conditions across years, such as water level changes or weather patterns. Recruitment fluctuations cause abundance of adult fish to vary from year to year, and the fishing quality varies accordingly regardless of the size limit.

From the above discussion, it is clear that Growth, Mortality, and Recruitment will interact to determine the success of a regulation. Fish populations with fast growth, low mortality, and high recruitment will produce outstanding catches of both quality and trophy-sized fish. Lake Istokpoga supports premier largemouth bass and speckled perch fisheries, and this probably occurs due to quality habitat resulting in rapid growth and good recruitment in most years. Lakes that are not commonly associated with trophy fish or have low catch rates of quality fish may suffer from slow growth, high mortality, low recruitment, or a combination of these factors. You can probably think of some of these as well in Highlands County.

For lakes with slow growth, the only practical tool for reducing the abundance of small fish and improving growth rates is angler harvest. For some bass fisheries, this is a tool that has declined in effectiveness due to a strict catch-and-release ethic among bass anglers. In a period of about the last 15 years, bass anglers have changed from harvesting most legal fish to a strongly entrenched catch-and-release angling ethic. Most recent creel survey data has shown that over 70% of bass caught are released, even when the fish were legal to keep. Anglers have had a strong influence on the conservation of our fishery resources and have improved the abundance of preferred-size fish in many systems through catch-and-release.

However, I have found that many anglers believe that strict catch-and-release will improve all bass populations, or that a 10-inch size limit will improve all speckled perch fisheries. Regulations should be tailored for the management objectives of each lake, and viable management options depend on the G, M, and R for each population. Just as importantly, angler harvest rates should also change depending on G, M, and R for each population! Some lakes will benefit from complete catch and release, but lakes with slow growth or abundant small fish would benefit from selective harvest to reduce the number of small fish and improve growth rates.

Slot limits are used to encourage harvest of small fish, improve growth rates, and protect the large fish in a lake. However, complete catch-and-release by anglers on lakes with a slot limit may result in too many small fish and reduced growth rates. This can potentially be detrimental to producing trophy fish. Selective harvest of smaller fish can actually help some fisheries by increasing the available food per individual, improving growth rates, yet protecting large fish from harvest to allow them to reach trophy size.

In summary, size limits are a tool for fisheries managers and have the potential to improve catch of large fish and total harvest. However, size limits will not improve every population! Fishery managers in Florida use specific regulations to improve fisheries that have rapid growth and good recruitment, such as the 15-24 inch slot limit on bass at Lake Istokpoga. Conversely, other fisheries with slow or moderate growth are often managed with more liberal size limits to allow anglers to harvest slower growing fish. The potential for success depends on G, M, and R for each population! If you want more information, contact your local Florida Fish and Wildlife Conservation Commission biologist to ask about conditions for individual lakes. Good fishing!